This invention relates to the production of III A - V B compounds in general and more particularly to an improved process for producing polycrystalline compounds of this nature.
It has been well established that compounds known as III A - V B compounds are useful in semiconductor devices. What is referred to by the designation III A and V B are those elements of groups III A and V B of the periodic table of elements.
Monocrystalline gallium phosphide is becoming increasingly important as a basic material for the production of luminescent diodes that emit light in the visible region. One method of obtaining such monocrystalline gallium phosphide uses, as a starting material, polycrystalline gallium phosphide.
Various methods have been developed to produce this starting material. In a process described by C. J. Frosch and L. Derick in the Journal of the Electrochemical Society, Vol. 108, p. 251 (1961), polycrystalline gallium phosphide is produced from white phosphorus and gallium at temperatures of between 1450.degree. and 1500.degree. C and at a pressure of between 6 and 35 atmospheres. This process is extremely difficult to carry out and is not of particular interest for industrial use. In carrying out the process, the strength limit of the quartz ampule is exceeded because of the necessary high pressure and high temperature occuring at the same time, which, as a result, causes an extreme pressure to be exerted on the ampule. That is to say, the conversion is carried out in an ampule set in an autoclave and as a result, the wide pressure range of 6 to 35 atmospheres makes it difficult to adjust the necessary counterpressure on the ampule since the internal pressure cannot be measured. The high emission frequency requires a small penetration of the eddy currents into the workpiece such as a graphite boat. In other words, in order to obtain a high temperature it must be greatly overheated locally. As a result of this and the high temperature of 1500.degree. C that exists in any case, a dark nontransparent coating is produced on the ampule wall, the coating consisting of gallium phosphide and carbon, thereby making optical measurement of temperature during the reaction impossible. In carrying out the reaction, the boat with the gallium is moved through the inductively-produced temperature region. Only with a subsequent second passage is it possible to obtain compact polycrystalline gallium phosphide which contains at the end of the synthesized bar free gallium.
Another method for the production of polycrystalline gallium phosphide is disclosed by S. J. Bass and P. E. Oliver in the Journal of Crystal Growth, Vol. 4, page 286 (1968). In their method, a reaction temperature of 1450.degree. C is used with the operative pressure required between 8 and 10 atmospheres. In this method, it is possible to work without an autoclave and a graphite tube sealed at both ends is used as the reaction vessel. Phosphorus vapor is led into the tube through holes and reacts therein with gallium to form the gallium phosphide. When operating, for example, such that the quartz ampule moves at a speed of 1cm/hr through the heated zone, gallium phosphide having a carbon content of approximately 1000 ppm is obtained. This contamination is not the only disadvantage in this method. In addition, sticking of the gallium phosphide to the reaction-tube is a problem as is the slow speed of travel through the heated zone, which speed is necessary in order to obtain compact gallium phosphide.
In another method disclosed by S. E. Blum, R. J. Chicotka, B. K. Bischoff, in the Journal of the Electrochemical Society, Vol. 115, page 324 (1968), gallium and phosphorus are converted to gallium phosphide at a temperature of 1500.degree. C and a phosphorus pressure of 5 to 24 atmospheres in an upright Bridgeman apparatus. In this method, the free gallium surface available is small and the time needed for complete conversion is correspondingly longer than in the case of a horizontal apparatus. The solidification speed of the completely molten gallium-phosphide is only approximately 1 cm per hour. Because of the long time required and the high temperature of 1500.degree. C used, a reaction occurs with the reaction vessel resulting in contamination of the semiconductor material and a detrimental change in the reaction vessel.
In another method disclosed in British Pat. No. 1,251,251, in German Offenlegungsschrift No. 1,911,715 and in an article by J. P. Besselers in Material Research Bulletin, Vol. 3, p. 797 (1968) gallium phosphide is produced from the elements at temperatures of 1000.degree. to 1200.degree. C and at a pressure of 1 atmosphere. The conversion, however, is very time-consuming with only 7 grams of GaP produced in 5 days. In addition, the reaction product contains gallium inclusions. In another process described in German Pat. No. 1,029,803, gallium phosphide is produced from the elements with the work done by the two-temperature process. This process is carried out at the melting point of gallium phosphide and the corresponding equilibrium vapor pressure. The crystal being formed continuously draws phosphorus from the melt, which is only delivered subsequently to a limited extent, as the vapor pressure drops causing it to come out of the vapor phase. The maximum possible quantities of polycrystalline gallium phosphide obtained through this method, i.e., 7 to 8 grams per batch, is no longer sufficient for present-day requirements.
Thus, it can be seen that all these prior art methods suffer from different disadvantages and there is a need for producing required quantities of gallium phosphides and the like using a simple and efficient process.